Solid propellants for rocket motors or the like include a high-energy fuel material, an oxidizer as required, and an elastomeric binder matrix in which the fuel material and oxidizer are dispersed and immobilized. Advantages of solid fuels in which an elastomeric binder material spatially immobilizes the fuel material and oxidizer are well known. The polymeric binder, however, represents a limitation in achieving maximum energy from a solid propellant in that the materials used in the binder burn with substantially lower energy than does the fuel material. It is desirable to minimize as much as possible the energy limitation that the binder material places on propellant formulations. Stated in an alternative manner, it is desirable that the binder burn along with the fuel material to release as much energy as possible. Achievement of binder energy value must be consistent with necessary characteristics of a binder, such as good elastomeric qualities and high mechanical strength parameters, particularly good stress and strain characteristics
Nitrate ester plasticizers, such as nitroglycerine (NG), trimethylol ethane trinitrate (TMETN) and butanetriol trinitrate (BTTN) are generally included along with the polymeric binder. The nitrate ester plasticizers enhance the elastomeric properties of the binder and are in themselves high-energy materials which contribute to the energy of the binder system and the propellant as a whole.
In selecting a binder polymer, the energy that is released when it is burned is an important consideration. If a nitrate ester plasticizer is included in the propellant formulation, it is a further requirement that the binder polymer be compatible with the nitrate ester plasticizer; otherwise the plasticizer will weep, i.e., flow and settle out from the binder. Certain polymers which have sufficiently high energies and would otherwise be desirable elastomers for binders cannot be used in certain binder systems due to their incompatibility or immiscibility with nitrate ester plasticizers. Hydrocarbon elastomers, for example, are inherently relatively high in energy; but are immiscible with nitrate esters. A potential binder polymer is a polyether prepared from tetrahydrofuran (THF), having high load-bearing capability and low glass transition temperature (T.sub.g); however, polymers prepared from THF polyethers are also immiscible with nitrate esters.
Propellant binders have been formed of polyethylene glycol (PEG) and polycaprolactone (PCP) and mixtures thereof. A PEG-based binder is more miscible with nitrate esters than is a THF-based binder, but insufficiently so for many propellant applications, and PEG-based binders are lower in energy than THF-based polymers. PCP-based binders have good miscibility with nitrate ester plasticizers, but PCP is substantially lower in energy than most other binder materials, tending to significantly reduce the energy value of the solid propellant as a whole.
In view of the above, there exists a need for new polymer-based elastomers having elastomeric and strength characteristics suitable for use as propellant binders, and which are inherently sufficiently high in energy and which are also miscible with nitrate esters.
U.S. Pat. No. 4,483,978 issued to G. E. Manser, the teachings of which are incorporated herein by reference, describes energetic copolymers and methods of making the same. The copolymers described in the '978 patent include copolymers of tetrahydrofurans and oxetanes having energetic pendent groups, such as azido, nitro and nitrato groups. The '978 patent teaches that the copolymers may be further copolymerized with a trifunctional substance to produce an elastomeric binder for a rocket propellant.
Copolymers, such as the THF/oxetane copolymers described in the '978 patent, are generally advantageous relative to homopolymers because the second mer unit, even in relatively small amounts, substantially reduces stereoregularity. Homopolymers having a high degree of stereoregularity exhibit substantial chain-folding, resulting in a compact structure which tends to be crystalline or highly viscous. Poly(THF) exhibits a high degree of crystalline structure. Poly(3-cyanomethyl-3-methyloxetane) is highly viscous, and thus, unsuitable for most propellant purposes.
Despite the general teaching in the '978 patent that THF/oxetane copolymers are generally useful in producing high-energy elastomeric binders for rocket propellants or the like, problems have been encountered in developing actual binder systems. Although there is ample description in the '978 patent of various copolymers formed from oxetane and tetrahydrofuran monomers, there is no example of a rocket propellant formulated with a binder that incorporates any such copolymer.
A problem which has been encountered when attempting to fabricate propellants from THF/oxetane copolymers is the tendency of the copolymers to have high viscosities. High-viscosity copolymers in an uncured propellant formulation with high-solids loading may be excessively viscous for processability. In preparing solid propellant formulations, relatively high levels of external plasticizers are used to decrease uncured formulation viscosity. Plasticizers which are energetic, such as nitrate ester plasticizers, are desirable in a propellant as they enhance the performance of the propellant. However, there is a limit to the amount of plasticizer which can be retained by any cured elastomer without the plasticizer migrating or "weeping" from the cured elastomer; hence, the problem of high polymer viscosity cannot necessarily be fully overcome through the use of external plasticizers.
The tendency to high viscosities of THF/oxetane polymers is attributable to a substantial degree of chain-packing. Chain-packing is the tendency for polymer molecules to closely associate with similar polymer molecules; this retards relative motion between the polymer molecules.
It would be desirable to provide THF/oxetane polymers having lower viscosities and further having other desirable characteristics which oxetane mer units may impart. For propellant formulations or the like, it is particularly desirable to have low-viscosity polymers which are miscible with relatively high proportions of nitrate ester plasticizers. It is further desirable that polymers for propellant formulations or the like have relatively high energies so as to contribute to the performance of the propellant.